Pulverized solid fuel has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The tangential firing method has many advantages, among them being good mixing of the pulverized solid fuel and air, stable flame conditions, and long residence time of combustion gases in the furnaces.
Systems for delivering the pulverized solid fuel (e.g., coal) to a steam generator typically include a plurality of nozzle assemblies through which the pulverized coal is delivered, using air, into a combustion chamber of the steam generator. The nozzle assemblies are typically disposed within windboxes, which may be located proximate to the corners of the steam generator. Each nozzle assembly includes a nozzle tip, which protrudes into the combustion chamber. Each nozzle tip delivers a single stream, or jet, of the pulverized coal and air into the combustion chamber. After leaving the nozzle tip, the single pulverized coal/air jet disperses in the combustion chamber.
Typically, the nozzle tips are arranged to tilt up and down to adjust the location of the flame within the combustion chamber. The flames produced at each pulverized solid fuel nozzle are stabilized through global heat- and mass-transfer processes. Thus, a single rotating flame envelope (e.g., a “fireball”), centrally located in the furnace, provides gradual but thorough and uniform pulverized solid fuel-air mixing throughout the entire furnace.
Recently, more and more emphasis has been placed on minimization of air pollution. In connection with this, with reference in particular to the matter of NOX control, it is known that oxides of nitrogen are created during fossil fuel combustion primarily by two separate mechanisms which have been identified to be thermal NOX and fuel NOX. Thermal NOX results from the thermal fixation of molecular nitrogen and oxygen in the combustion air. The rate of formation of thermal NOX is extremely sensitive to local flame temperature and somewhat less sensitive to local concentration of oxygen. Virtually all thermal NOX is formed at a region of the flame which is at the highest temperature. The thermal NOX concentration is subsequently “frozen” at a level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NOX concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NOX derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NOX is highly affected by the rate of mixing of the fossil fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NOX concentration due to fuel nitrogen is typically only a fraction, e.g., approximately 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fossil fuel. From the preceding, it should thus now be readily apparent that overall NOX formation is a function both of local oxygen levels and of peak flame temperatures.
Although the pulverized solid fuel nozzle tips of the prior art are operative for their intended purposes, there has nevertheless been evidenced in the prior art a need for such pulverized solid fuel nozzle tips to be further improved, specifically in the pursuit of reduced air pollution, e.g., NOX emissions. More specifically, a need has been evidenced in the prior art for a new and improved low NOX pulverized solid fuel nozzle tip for use in a tangential firing system that would enable more flexibility in the control of undesirable emissions such as nitric oxides.